The present invention is related generally to a method of and system for producing metal castings and, in particular, an improved method of and system for producing aluminum engine blocks and the like having thin cylinder walls which are substantially free of defects.
A variety of casting processes and materials have been proposed in the automobile industry for manufacturing engine blocks. One material commonly utilized is cast iron. Cast iron has gained wide acceptance over the years because it is relatively easy and inexpensive to cast as well as to subsequently machine the same. Also, a variety of different casting methods are used for casting blocks and these include sand casting followed by permanent die casting; low pressure die casting; and, high pressure die casting. Manufacturers are attempting to meet the demands for lighter, more compact and higher performance engines, and aluminum is generally favored for these purposes. Different techniques are used for producing cast aluminum blocks. One known process includes use of a low pressure die casting system, another is an evaporative casting process, such as the so-called lost foam process that utilizes sand dies and STYROFOAM (expanded polystyrene) filters; the latter of which evaporates. Yet other known processes include high pressure die casting, gravity casting, and sand casting.
The newer engine blocks have closer dimensions and tolerances and thin wall sections. One area of concern is the location of the cylinders because they are positioned to be in very close proximity to each other, and hence have relatively thin walls therebetween. Because of these very small spaces between the cylinders there is a possibility that portions of the aluminum cylinder wall will prematurely solidify before the entire block and, thus create areas having microscopic cracks.
For a variety of reasons, it is desirable to utilize cylinder sleeves made of cast iron in aluminum engines. One known high pressure die casting technique for producing such blocks does so by inserting the sleeve in the mold and subsequently mold the engine block therearound.
Disadvantages of the foregoing techniques in dealing with aluminum engine blocks are that microscopic defects can be formed at zones of non-uniform solidification or zones of contact with different materials (e.g. cast iron sleeves). Also, it is possible for the core pins used in the mold for forming the cylinders to distort thereby forming residual stresses in the mold not to mention damage to the bore pin. The noted defects to the engine are not detected easily and often show up only after the block is machined. As a consequence, there are ongoing efforts to improve upon the manufacturing of engine blocks having thin walls and especially such blocks made of aluminum or other similar materials.